题目

We would like to wish all our readers a wonderful winter break. Our January magazine is now in the shops and available digitally. We’re looking forward to sharing more adventures and discoveries with you in 2014, including: At a crossroads in the Atlantic As the population of Ascension Island rises up to mark the 200th anniversary of British rule, Fred Pearce wonders what the future might have in store for this strange part of land. Photostory: On the road again A selection of images from an exhibition opening this month at the Royal Geographical Society go hand in hand with M Aurel 如图所示，建筑工人用滑轮组提升重为180N的泥桶，不计滑轮与轴之间的摩擦及绳重．若工人在5s内将泥桶匀速提高了2m，滑轮组的机械效率为80%．求：（1）工人使用滑轮组时做的有用功、拉力做的总功．（2）工人拉绳子的力．（3）如果用这个装置同时提3个同样的泥桶，工人拉绳子的力为多少？